In 1588 he attained his majority, and joined the league of Christian Princes against the Turks. The obvious danger of such a course caused no small anxiety in the principality, and the diet of Torda (today: Turda, Romania) even went so far as to demand a fresh coronation oath from Sigismund. Upon his refusal to render it, the council members threatened him with deposition. Ultimately Báthory got the better of his opponents, and executed all whom he got into his hands (1595).[3]

On August 13, 1595 at the Battle of Călugăreni near the Neajlov river, Michael defeated a Turkish army led by Sinan Pasha. Despite the victory Michael, having too few troops to continue the war, retreated toward Transylvania. Joining Sigismund Báthory's 40,000-strong army led by István Bocskay, they liberated Târgovişte (October 8, 1595), Bucharest (October 12, 1595) and Brăila. Wallachia was liberated on October 29, 1595.

In April 1598 Sigismund resigned as Prince of Transylvania in favor of Emperor Rudolf, reversed his decision in October 1598, and then resigned again in favor of Cardinal Andrew Báthory, his cousin. This allowed Transylvania to fall under the influence of the King of Poland. Michael the Brave reestablished an alliance with Emperor Rudolf, began a campaign against Andrew on October 5, 1599, and became Prince of Transylvania in November 1599, while the Habsburg general Giorgio Basta entered Transylvania from the west at the same time.